The electrolytic production of hypochlorite from diluted brines of alkali metal chlorides, e.g. of sodium hypochlorite by electrolysis of aqueous solution of sodium chloride or of sea-water, is one of the most common processes in the domain of industrial electrochemistry. The production of hypochlorite is always accompanied by the generation of various by-products deriving from the oxidation of chlorides (generally grouped under the name of “active chlorine”) and in some cases of oxygenated species such as peroxides, most of which have a very limited lifetime; for the sake of brevity, in the present text the whole of such products in aqueous solution, mostly consisting of alkali metal hypochlorite and hypochlorous acid in a ratio mainly depending on pH, is indicated as hypochlorite. In many applications it is precisely the intrinsic lability and short shelf-life of a few very active species that makes the in situ production of hypochlorite attractive, allowing an immediate use of the product solution. This is especially true in the medical/hospital field (sterilisation of gauzes or surgical tools), in the hospitality/service industry (white tissue disinfection, pre-treatment of water supplied to showers and sanitary ware), in food and beverage (treatment and packaging of solid and liquid foodstuff), in laundry and in the farming and meat industry. In many of these environments it would be desirable to have a ready-to-use hypochlorite generation system available, as flexible as possible in order to be able to respond to different needs with one single device. For example, in the field of industrial farming, hypochlorite might be requested at different volumes and concentrations for disinfection of the relevant machinery or for treating the animal skin, and likewise in a hotel environment distinct hypochlorite solutions might be used for linen bleaching or for the disinfection of water directed to showers and sanitary ware; it would therefore be useful to provide a device allowing to set the characteristics of the required product according to the needs of the moment. The simplest and most effective way to produce hypochlorite electrochemically is the electrolysis in cells of the undivided type, with electrodes of various shapes and geometry, for example with interleaved planar electrodes. In an electrolytic cell, hypochlorite production takes place by anodic oxidation of chloride, with hydrogen being concurrently evolved at the cathode; when the chloride solution to be electrolysed contains sensible amounts of calcium or magnesium ions, such as the case of civil water chlorination, the natural alkalinisation of the electrolyte in the proximity of the cathode surface causes the local precipitation of carbonate, which tends to deactivate the cathodes and force them to be put out of service after some time. Among the various solutions proposed to obviate this problem, a very effective one consists of submitting the electrodes to cyclic potential reversal, alternating their use as cathodes and as anodes. In this way, the carbonate deposit which settles on the surface of an electrode under cathodic operation is dissolved during the subsequent operation as anode, when the reaction environment tends to get acidified. Since the hydrogen evolution reaction takes place at a sufficiently moderate potential on many metallic materials, the electrodes of an electrochlorinator which has to work under alternate electrodic polarisation are activated with a catalyst designed to maximise the efficiency of the more critical hypochlorite generation anodic reaction. The functioning of the electrodes in alternate polarisation conditions allows operating with good efficiency while keeping the electrode surface sufficiently clean from insoluble deposits; nevertheless, the cathodic operation under hydrogen evolution of electrode configurations of this kind entails a less than optimal operative lifetime, because the adhesion of the coating to the substrate tends to be hampered in these conditions. The deactivation mechanism of this type of electrodes, fundamentally due to the detachment of the catalytic layer from the substrate, brings about a sudden failure with no significant premonitory sign. in order to prevent serious inconveniences, an estimation of the residual lifetime of electrodes in a cell is often carried out on a statistical basis, so as to proceed with their replacement before a quick and irreversible failure occurs. Since the deactivation of electrodes working under this kind of operative conditions is affected by several factors, its variability is rather high, and keeping a sufficient margin of safety implies the replacement of electrodes which might have been functioning for a significant residual time. Such variability, which is high per se also for cells functioning at constant working conditions, becomes almost uncontrollable for cells subjected to working cycles at always changing conditions, to be able to rapidly manufacture hypochlorite solutions of variable volumes and concentrations according to the different needs. In this case, even a significant historical data collection on many cells is not very useful in predicting the residual lifetime of electrodes, which is strongly dependant on the type of solicitation they have been subjected to, in its turn affected by the operative needs of the individual user.
It has been thus evidenced the need for providing a new system of electrochemical generation of hypochlorite characterised by an enhanced flexibility of use and at the same time by the possibility of predicting the deactivation of the electrodes and the consequent need to schedule a replacement intervention thereof some time in advance.